Method of prospecting for mineral deposits having radioactive gaseous decay products

ABSTRACT

Prospecting, particularly prospecting for uranium, thorium and the like radioactive ore deposits having a gaseous decay product which diffuses through the earth&#39;&#39;s structure and becomes windborne.

United States Patent METHOD OF PROSPECTING FOR MINERAL DEPOSITS HAVING RADIOACTIVE GASEOUS DECAY PRODUCTS 4 Claims, 12 Drawing Figs.

US. Cl.....' 250/83, 250/83.6

Int. Cl G0lt 1/16 Field of Search 250/435 R,

83 SA, 83.6 S

[56] References Cited UNITED STATES PATENTS 2,906,882 9/1959 Merritt 250/83 SA 3,008,046 11/1961 Carpenter 250/83.6 S 3,056,886 10/1962 Glaude et 81.... 250/435 R 3,143,648 8/1964 Bradley et a1... 250/336 S 3,158,741 11/1964 Skvarla 250/435 R 3,180,983 4/1965 1-1311, Jr. et al 250/83 SA Primary Examiner-Archie R. Borchelt Attorney-Semmes & Semmes ABSTRACT: Prospecting, particularly prospecting for uranium, thorium and the like radioactive ore deposits having a gaseous decay product which diffuses through the earths structure and becomes windborne.

PATENTEUSEP28L9H 3,609,363

SHEET 2 [IF 4 BY semmesandsemmes ATTORNEYS METHOD OF PROSPECTING FOR MINERAL DEPOSITS HAVING RADIOACTIVE GASEOUS DECAY PRODUCTS BACKGROUND OF THE INVENTION 1. Field of the Invention Uranium, thorium and the like radioactive deposits exude a gaseous decay product which diffuses through the earths structure into the atmosphere. Conventional prospecting techniques include transversing the earth's structure with a scintillation detector or the like device which counts gamma activity. Shortcomings of these techniques include relatively short operational range and blocking of counting by a relatively small amount of over burden in the earths surface. Another shortcoming of such techniques resides in the necessity for line of sight counting of radioactivity. Thus, the prospector was required to be physically present on the earth s surface at which the radioactive ore is located. There has been no prior art attention given to the use of mobile vapor phase tracking or the combination of such tracking with combining meteorological considerations. For example, no earlier inventors have proposed horizontal tracking of density flow of gaseous decay products downwind of an ore deposit, then marking of the ore deposit as density flow and diffusion vertically of radioactivity through the earths structure coincide.

2. Description of the Prior Art Typical prior art prospecting techniques include U.S. Pat. to Herzog, to Ford, No. 2,956,164. In both prior patents consideration is given to the use of airborne scintillation counters. Ford conducts an aerial survey by means of a scintillation counter, the number of counts detected by the counter being inversely proportional to the altitude of the aircraft since gamma ray transmission decreases as the square of the altitude. l-Ierzog, on the other hand, determines gamma-ray intensity at several points in an overburden formation, so as to locate or pinpoint the ore deposit. In FIGS. 8-10 Herzog suggests airborne measuring taking into account aircraft elevation and speed.

See also Falk, U.S. Pat. No. 3,373,282, which teaches measuring the radiation dose rate within the atmosphere by means of a gas density meter.

However, neither patent remotely suggests mobile tracking of gaseous decay products by azimuthly charting a sector of density flow of gaseous decay product, then horizontally tracking the density flow towards the ore deposit and marking the ore deposit as density flow and diffusing vertically from ore deposit coincide.

DESCRIPTION OF THE INVENTION Applicant prospects for mineral deposits of the type having a gaseous decay product which diffuses through the earths structure into the atmosphere by initially sensing the gaseous products and its decay product in the atmosphere; discriminating between density flow of gaseous decay products and ambient decay products, horizontally tracking the density flow towards the ore deposits and marking the ore deposits as density flow and diffusion of radioactivity through the earth s structure from said ore deposit coincide. Refinements of invention include initially conducting a line survey downwind of the area being prospected; outlining katabatic flow as a function of topography, counting the ratio of alpha to gamma activity as a function of age of the radioactive decay products of the initial gas so as to determine the age of the cloud being measured, dust sampling during tracking and capping of the earth's structure adjacent the diffusion of radioactivity through the earths structure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing diffusion of gaseous decay products from the ore deposit through the earths structure and into the atmosphere where it becomes airborne;

FIG. 2 is a graph-depicting formation of the low level temperature inversions under which the desired katabatic or density flow conditions occur;

FIG. 3 is a schematic view showing katabatic wind drainage within a given topographic sector;

FIG. 4 is a chart showing the development of slope winds within a valley of the topographic sector shown in FIG. 3;

FIG. 5 is a schematic illustration of tracking upstream" ac cording to the katabatic wind flow shown in FIGS. 3 and 4;

FIG. 6 is a schematic view of line survey tracking downwind of an area being prospected;

FIG. 7 is a schematic view of fixed point sensing and tracking under various wind conditions;

FIG. 8 is a graph showing determining of cloud age by counting the ratio of alpha to gamma disintegration rates;

FIGS. 9A, 9B and 9C are schematic views showing sampling and capping of the earth s structure about a projected ore deposit and measuring radioactivity as the function of diffusion vertically through the earth s structure.

FIG. 10 shows mapping of said gaseous diffusion, according to FIG. 9.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT Applicants method is based upon the detection and tracking of airborne clouds of radioactive material in the atmosphere, these clouds arising from the radioactive decay of elements contained in the ores in the ground to produce a gaseous decay product which diffuses through the earths structure into the atmosphere. The airborne gas and its decay products can then be detected by means of observations over the ground surface and at points removed from the deposit; and these observations can then be related to the location of the deposit by meteorological considerations. Because of the diffusivity of the gaseous product through the ground, detection of deposits at depth, without recourse to drilling becomes possible, and because of the travel of the gaseous cloud and its decay products with the wind after diffusion into the at mosphere, detection of deposits can be accomplished at distances which are considered great by the standards of normal prospecting methods. Furthermore, by application of the principles of micrometeorology, advantage can be taken of the occurrence of conditions under which confluence of wind from vast region prevails, rendering it possible thereby to scan comprehensively the entire region by measurements made at a single point.

An essential element in the successful employment of this technique is the application of meteorological knowledge, concerning cloud travel and dilution, to govern the observational regime and the interpretation of measurements. To achieve success, measurements must ordinarily be made under those conditions (which commonly occur) that give rise to concentration of the products in the lower atmospheric layers. This is readily done by taking advantage of low level temperature inversions arising particularly in FIG. 2. There, the frames t,-t,, are designated to illustrate the formation of cool, dense air layers adjacent the earths surface, beginning at sunset. Further, under these conditions katabatic, or density, airflow occurs and under synoptic anticyclonic conditions with weak geostrophic pressure gradients the airflow will be determined by the topographic relief. The result is that confluence of katabatic flow occurs in precisely the same manner as the hydrologic drainage of a watershed, and entire valley systems may be surveyed by single measurements at the valley mouth. Successive measurements upvalley will serve to progressively eliminate subsidiary valley systems from consideration or identify them as major contributors. Continued upvalley study can then identify the region of the cloud source and hence, the responsible ore.

Strong relief is not required for successful application and other variations of survey technique are readily employed even for very flat country.

For example, continuous monitoring at a fixed point over sufficiently long time to characterize all wind directions will provide a survey of the surrounding countryside for distances of the order of miles. Similarly, continuous measurements along a line survey (as following a road) will scan the country to the upwind side of the line. In all cases, however, it is necessary that competent consideration be given to the meteorological factors which govern the efficiency with which the initial emanations are concentrated and with which subsequent travel and turbulent dilution processes occur.

Other refinements may also be applied. For example, under certain conditions, i.e. a well defined single course, the ratio of alpha to gamma radiation from atmospheric samples provides an estimate of the age of the cloud being measured. This information together with the wind speed permits an estimation of the distance from the point of measurement to the point of origin, projected back along the wind trajectory.

Since uranium and thorium each lead to a gaseous intermediate decay product (radon and thoron respectively) the method is applicable to both types of ores and differentiation between them may be made in the same observation on the basis of half-life characteristics of the decay products.

The method of measuring the concentration of decay products at a point in the atmosphere is based on the fact that upon decay of, for example, radon atoms, the recoil ions become attached to particles of submicroscopic dust. A sample of this dust is obtained from the atmosphere in a concentrated form suitable for radiometric counting by filtration or electrostatic precipitation. (Both methods were used equally well in the experiments accomplished). The sample is then counted, primarily for alpha activity, since except for occasional spurious results, ascribable to atomic weapons tests, this activity can only come from the ores of interest. It is therefore a unique indicator of ore in contrast to the numerous complicating factors such as background changes, cosmic radiation, mass effect, etc. which enter into conventional gamma radiation measurements.

After detection and general localization of an area of interest, further adaptation of the technique may be employed to outline more precisely the ore body itself. By capping the emanating soil surface so as to trap the gaseous products, at a succession of points over the area in question, and measuring the content so derived, contour analysis of the results will aid in ore body definition.

This method of prospecting is also considered applicable to any other mineral deposits which give rise to gaseous emanations capable of diffusion into the atmosphere, for example natural gas and petroleum.

EXPERIMENTAL The following examples are extracted from the field experiment notes to indicate the nature of the experimental verification completed to date and the characteristics and potential worth of the technique.

EXPERIMENT 1 Measurements of airborne activity were made by sampling on a filter for 5 minutes and counting alpha activity for 5 minutes using an alpha scintillation detector and scaler. The deposit had been outlined and assessed by means of drilling and shown to be approximately 200 X 500 feet in extent, and about 4 feet thick at 100 feet depth. The average ore grade was 0.35 percent. Meteorological conditions were weakly anticyclonic with almost no gradient, and developing low level radiation inversion. Local terrain was quite flat for several miles with a very shallow drainage into an adjacent major valley system. Measurements of alpha activity were made at intervals along a line perpendicular to the wind direction and about 0.7 miles downwind of the deposit. During approach to the downwind projection of the deposit counts in the range of 6-30 were obtained. Upon encountering the projection, the count rose abruptly to 90-95. Because of the presence of drill holes serving as potential radon release vents, the experiment is inconclusive in that it does not prove detection at depth. The ability to detect a relatively weak cloud as an identifiable entity under proper conditions was amply demonstrated.

EXPERIMENT 2 A series of measurements similar to the above was made at a fixed point in the main valley stream approximately 10 miles downwind of the known Ambrosia Lake deposits. These measurements showed counts in the range of 5-15 during typical convective conditions even when the wind was directly from the deposits. Counts were in the range of 25-50 when the wind was upvalley (toward the deposits) under conditions which were as stable as possible consistent with upvalley flow. Under katabatic flow, however, the counts were repeatedly observed to be in the range 150-250.

Moving the point of observation up the valley for 3 miles continued to show values in this range, suggesting the activity originally observed had come from great distances and presumed to be from the Ambrosia Lake region at a distance of 10 miles.

These experiments serve to demonstrate the unequivocal detection (very high count/background ratio) at a distance of greater than 3 miles and probably of the order of 10 miles from important deposits.

EXPERIMENT 3 Since the high counts are a result of the concentrating effect within a stable atmosphere it seemed logical to anticipate a decrease in count with height. In order to obtain an appreciation of the magnitude of this effect, advantage was taken of an insular spur longitudinal to the main Ambrosia valley axis and a series of measurements taken at various heights along this spur during well-developed katabatic flow in the main valley. These measurements showed a decrease from counts of 150-200 at the valley floor to about 60 at -200 feet above the valley floor.

These experiments demonstrated that in a valley system being fed by major deposits the depth of effect is sufficiently great even under strong katabatic conditions, that aerial surveys appear feasible.

EXPERIMENT 4 Since radon emanates from all ground surfaces, the question arises whether the high counts observed in experiments 2 and 3 do not reflect solely the concentrating effect of a stable atmosphere over a great area of normal background rather than being associated with the known deposits. To obtain further indications concerning this possibility, a series of observations was made several miles up the mouths of a series of major side valleys which empty into the Rio Grande Valley; the measurements were made under similar stability and katabatic flow conditions to those in Ambrosia Valley but in none of five such cases were counts of similar magnitude encountered-being in the range of 20-50.

Thus it is concluded from this series that the high counts when observed as in the Ambrosia Valley are not simply a reflection of the concentrating effect of the atmosphere but are associated with extensive ore deposits.

Manifestly, various instrumentations may be employed in sensing, tracking and marking according to applicant's method without departing from the spirit of invention.

Iclaim:

1. Method of prospecting for mineral deposits of the type having radioactive gaseous decay products which diffuse vertically through the earth's structure into the atmosphere comprising:

A. sensing said radioactive gaseous decay products in the atmosphere under katabatic conditions determined by topographic relief and temperature inversion;

B. charting an asimuthal sector of density flow of said gaseous decay products above the earth's surface;

C. horizontally tracking said density flow towards the deposit from which it has diffused; and

D. marking said deposit within the earth, as said density flow and diffusion vertically from said deposit coincide.

said density flow during tracking as a function of distance to said mineral deposits by counting the ratio of alpha to gamma disintegration rates in said gaseous decay products.

4. Method of prospecting as in claim 3, including vertically tracking said density flow, while counting radioactivity as a function of the magnitude of said density flow. 

2. Method of prospecting as in claim 1, wherein said sensing is done from a fixed point with respect to proposed mineral deposits, said tracking is correlated with ambient wind conditions, and including marking of a projected deposit from said fixed point, radioactive density flow and diffusion vertically being projected as a function of ambient wind and the age of radioactivity in the flow being tracked.
 3. Method of prospecting as in claim 1, including aging of said density flow during tracking as a function of distance to said mineral deposits by counting the ratio of alpha to gamma disintegration rates in said gaseous decay products.
 4. Method of prospecting as in claim 3, including vertically tracking said density flow, while counting radioactivity as a function of the magnitude of said density flow. 